The present invention relates generally to addressable subscription television decoders and more particularly concerns apparatus for increasing the probability that a broadcast address code will be properly recognized by such a decoder.
Subscription television systems employing addressable decoders are well known in the art. Exemplary of such systems is that disclosed in U.S. Pat. No. 4,222,068 to Thompson. In the Thompson system, as well as in other known similar systems, television signals are broadcast in a scrambled form from a head-end facility to each of a plurality of system subscribers. Various data signals such as subscriber address codes, subscriber authorization codes and program codes may be encoded in the vertical intervals of a broadcast television signal at the head-end facility so as to enable head-end control of the program viewing authorization of each system subscriber. In particular, each system subscriber is provided with a decoder having a unique address and including a memory for storing data reflecting the subscriber's program authorization status. In order to initialize or update the program authorization status of any particular subscriber, that subscriber's unique address code together with an authorization code representing the desired subscriber program authorization status is encoded in the vertical interval of the transmitted television signal. The subscriber's decoder is responsive to the transmitted address code for storing the associated program authorization code in memory whereby the entire authorization procedure is completed from the head-end facility.
The program codes encoded in the vertical intervals of the transmitted television signals identify one or more program categories reflecting the programming content (e.g. sports, news, movies, etc.) of the accompanying signal. The received program codes are compared with the stored program authorization status in each subscriber's decoder to determine whether or not the subscriber is authorized to decode the accompanying television signal. If the comparison indicates that a subscriber is authorized for viewing a particular television signal, an unscrambling circuit in the subscriber's decoder is enabled for unscrambling the television signal, the decoder otherwise coupling the received television signal in a scrambled and thereby unviewable form to the subscriber's television receiver.
One known technique for scrambling a transmitted television signal involves suppressing the horizontal sync pulses during the video fields. In this scrambling technique, the sync pulses occuring during the vertical intervals of the television signal are normally not suppressed in order to facilitate data reception by the decoder. While this technique provides for adequate video scrambling, it has the undesireable characteristic of reducing the probability of proper data recognition by the decoder.
More specifically, in order to process the data encoded in the vertical intervals of a transmitted television signal, the received signal must initially be converted to a baseband form. In order to maintain proper operating signal levels, an AGC circuit is typically used to control the gain of the IF stage of the decoder in response to the level of the detected baseband video signal. In particular, the AGC circuit is gated for sampling the horizontal sync tip levels of the detected video signal for adjusting the gain of the IF stage to maintain the detected signal at a substantially constant level. In the foregoing example of a scrambled television signal characterized by fields of suppressed horizontal sync pulses, the AGC circuit will therefore tend to increase the gain of the IF stage in an attempt to compensate for the reduced horizontal sync tip levels of the detected video signal. As a consequence, the non-suppressed sync pulses characterizing the vertical intervals of the transmitted video signal are subjected to increased gain such that the detected sync pulses are produced at excessive signal level. Since recognition of the data signals encoded in the horizontal lines of the vertical intervals of the transmitted signal is generally referenced in relation to the vertical sync pulses, the probability of proper data signal recognition by the decoder is substantially reduced due to the abnormal signal levels of the vertical sync pulses. As a result of this decreased data signal recognition probability, an excessive period of time may transpire before the data, which is redundantly transmitted on a cyclical basis, is properly recognized by a given decoder. That is, each time that a transmitted address code identifying a particular decoder is not properly recognized thereby, the decoder must wait a certain period of time before its address code is retransmitted. This effect can therefore substantially delay initialization of or changes in decoder program authorization status, such delays being highly undesireable from the viewpoint of both the system operator as well as the system subscribers.
It is therefore a basic object of the present invention to provide an improved addressable decoder for a subscription television system.
It is a more specific object of the invention to provide an addressable decoder for a subscription television system which is characterized by the ability to properly recognize transmitted data to a high degree of probability.